A variety of systems exist for the digital distribution of content (i.e. multimedia such as movies, televisions shows, and other combinations of audio and/or video information). The term online content distribution system typically refers to a system that enables the digital distribution of content via a network. The content can be downloaded as a multimedia file or streamed to a playback device. The term off-line content delivery system refers to a system that involves the delivery of digital content using some form of fixed media such as an optical storage device, or a portable media drive. In an off-line content delivery system, the delivery of the content relies upon the physical transportation of the fixed media. In many instances, off-line content delivery systems can overcome bandwidth limitations imposed by existing network infrastructure. Accordingly, companies such as Netflix, Inc. of Los Gatos, Calif. have achieved considerable success delivering DVDs to subscribers via the postal service.
A number of Video on Demand (VOD) systems exist for distribution of on-demand content via cable, IPTV, and/or satellite television systems. These systems typically use a Conditional. Access or CA system to encrypt the VOD content for playback on authorized playback devices. The CA systems often protect the content using processes including encryption processes and provide an encrypted Entitlement Control Message (ECM), which can be used by an authorized playback device to access the protected content. In many systems that utilize ECMs, the CA system also generates a separate Entitlement Management Message (EMM), which enables a playback device authorized to playback the content to decrypt the ECM and playback the on-demand content.
Many CA systems use symmetric encryption algorithms such as 128-bit AES or RC4 encryption to pre-encrypt on-demand content prior to storing the encrypted content on dedicated video-on-demand (VOD) servers, or downloading the encrypted content to client devices in “push VOD” applications. Symmetric-key encryption algorithms are a class of algorithms for cryptography that use trivially related, often identical, cryptographic keys for both decryption and encryption. The encryption key is trivially related to the decryption key, in the sense that they may be identical or there is a simple transformation to go between the two keys. The keys represent a shared secret between two or more parties that can be used to maintain the privacy of the encrypted content. Some content owners regard the use of symmetric encryption algorithms as representing a single point of compromise for hackers.